Metal coating composition and method of coating steel



U te S a e Pate 2,987,428 METAL COATING COMPOSITIONAND METHOD OF COATINGSTEEL Ferdinand P. Heller, Philadelphia, and Roger L. Beamon, Lansdale,Pa., assignors to Amchem Products, Inc., a corporation of Delaware NoDrawing. Filed Mar. 14, 1958, Ser. No. 721,340 3 Claims. (Cl. 148-615)This invention relates to the art of, coating metal and especially tothe provision of improved compositions useful for this purpose as wellas to the provision of an improved method for coating steel. Moreparticularly the invention relates to the art of applying asubstantially amorphous pyrophosphate coating on metal surfaces withoutdepositing thereon any substantial overburden of powder.

As is known to those skilled in the art attempts have been made toincrease the corrosion resistance of metal surfaces and prepare them forthe subsequent application of a final siccative finish by treating themwith acid pyrophosphates. However, the aqueous acid pyrophosphatesolutions heretofore employed for this purpose involve certain inherentdisadvantages or difliculties. One of these arises by virtue of therapid reversion of acid metal pyrophosphates to orthophosphates whendisso'lvedin aqueous solutions even when the solutions are employed atordinary living or room temperatures. Furthermore, if pyrophosphatesolutions which are relatively stable at room temperature are employedthey still lead to difficulties of a similar nature because, whenheated, they change readily to the ortho form. Still further, additionaldifliculties are encountered with the use of pyrophosphates arising fromthe fact that more free acid is produced from pyrophosphates byhydrolysis than is thecase where 'a similar amount of a corresponding'orthophosphate is employed.

In efforts to overcome the difiiculties referred to a number ofproposals have been made by those skilled in the art, one of which isrepresented by U.S. Patent No. 2,067,- 007. This patent suggests thatthe problems referred to can be overcome through the use of so-calledbalanced solutions containing a pyrophosphate salt of certain metalssuch as zinc acid pyrophosphate. Unfortunately, how- 'ever, solutions ofthis type tend to form copious quantities of sludge as well as toproduce powdery and nonadherent coatings on the surfaces of the metalbeing treated. In order to avoid decomposition of the pyrophosphate thepatent just mentioned recommends that, in addition to pyrophosphate, thesolultion should contain dissolved orthophosphate. q I

The difiiculties referred to which have been encountered heretofore inmaintaining balance (i.e., maintaining a proper relationship betweenfree and total" acidity) as well as those involved in maintainingsuitable ortho to pyro phosphate ratios have prevented the process fromachieving commercial'success. This is emphasized by the fact that thesolutions of the prior art required: 1) virtual saturation with respectto a solid, heavy, metal pyrophosphate, (2) definite control'of the freeacid content of the pyrophosphate solution and (3) the simultaneouspresence of orthophosphate and pyrophosphate to insure solutionstability. 7

In sharp contrast to the foregoing the present invention is notdependent upon any of these criteria for its success. Rather it consistsin the provision'of means for applying a new and superior type ofpyrophosphate coating from solutions which 'are simple in compositionand easy to maintain in operating condition. In other words, ourinvention makes it possible toernp loy a composition and process whichare almost completely free of the problems incident to the variables ofconcentration, temperature, etc. as will further appear.

Patented June 6,- 1961 ice With the foregoing in mind the objects of theinvention will be better understood and they may be said to include theprovision of a phosphating solution which remains substantially free ofsludge and which is capable of producing uniform power-free andsubstantially amorphous pyrophosphate coatings on the surfaces of steelas well as the provision of a solution and a method which are capable ofproducing very smooth and exceptionally flexible pyrophosphate coatingson the surfaces of metal objects. A concomitant object consists in theprovision of a process which is capable of forming pyrophosphatecoatings on metal surfaces which coatings permit the subsequentapplication of appreciably thinner layers of a final siccative surfacefinish having a higher degree of gloss than is ordinarily obtainablethrough the application of a surface finish over a conventionalphosphate coating.

Additional objects of our invention include the provision of a processwhich is easily controlled and which does not require the use of socalled balanced phosphate solutions as well as the provision of aphosphate solution which can be simply and inexpensively prepared andeasily controlled and replenished.

A still further object is the provision of solutions which do not formobjectionably large quantities of insoluble sludge or encrustations onthe equipment used.

How the foregoing together with such other objects which may appearhereinafter are attained will now be described.

Our invention is based upon the discovery that compositions containing,as their essential coating ingredients, pyrophosphate selected from theclass consisting of sodium, potassium, ammonium and lithiumpyrophosphate, ferric ion and oxidizing accelerating agents which arecapable of oxidizing ferrous iron to ferric iron, are ideally suited forthe formation of an apparently'amorphous pyrophosphate coating on metalsurfaces without the pres ence of zinc, manganese or other so-calledcoating metal phosphates. The solutions and process of the inventionyield unusually smooth, uniformly coated and substantially amorphouscoatings without the formation of a substantial overburden of powder.

In general the solutions of our invention contain from 0.05 to 12 or 13%of a pyrophosphate depending upon the type chosen and limited by themaximum solubility of the pyrophosphate in aqueous solutions. However,the preferred concentration of the pyrophosphate has been found to befrom about 1 to 5% since entirely satisfactory coatings, which aresmooth, uniform in appearance and amorphous in character, are readilyobtained from solutions containing the preferred amount of coatingproducing ingredient. At concentrations of less than 1% the length oftime required to produce the desired coating is increased and atconcentrations of less than 0.05% no useful coating is produced inreasonable treating times. No increase in the speed of the coatingreaction is obtained if concentrations are greater than 5%, although nodetrimental effects are encountered at higher concentrations. Therefore,in the interest of economy, we generally prefer to use not more than 5%of the pyrophosphate.

The minimal amount of ferric iron which must be incorporated into thepyrophosphate bath in order to insure production of the desiredamorphous coating is 0.0008 part for each part of pyrophosphatecalculated as P 0 Above this quantity, we have found that the desiredcoating results can be obtained where an amount of ferric iron isincluded in the bath all the way up'to the limit'of this solubilitywhich, in these pyrophosphate solutions, is approximately 0.06 part foreach part or no harm. Use of less than 0.0008 part ferric iron for eachpart P may result in no coating being formed on the surface of the workwhen the pyrophosphate solution is applied in a spray process.

The accelerator used in the pyrophosphating solution must be selectedfrom the class ofoxidizing agents which are capable of oxidizing ferrousto ferric iron. Typical examples are chlorates, nitrites and per oxygencompounds and they should be added in an amount chosen so as to give anefiect equivalent to from 0.2 gram to 10 grams of chlorate per 100milliliters of coating solution. The preferred accelerator is chloratealthough entirely satisfactory coating results are obtained from the useof one or a combination of the accelerators previously mentioned. Use ofless than 0.2 gram of chlorate or equivalent per 100 mls. of coatingbath is not efiective and results in a non-uniform coating of the metalsurface. Conversely, use of more than 10 grams/100 mls. of coating bathor equivalent amount of other accelerator oflers no additional advantageover those obtained at the 10% level and at times may actually bedetrimental.

Other accelerators such as nitrate, may be used in conjunction with theprimary accelerators previously specified and where used the amount ofprimary accelerator may be reduced. For instance, as little as 0.2 gramof NaNO per liter of coating solution may be used when 7 grams/liter ofNaNO are used.

The pH of the bath is also important and, in general, should not be solow that the solution contains free pyrophosphoric acid. We have foundthat a pH range of 3.2 to approximately 6.0 produces acceptableamorphous pyrophosphate coatings. However, optimum results are attainedwhere the pH ranges from 3.8 to 5.5 for which reason we prefer tooperate within this range. Use of a pH below 3.2 results in theformation of free pyrophosphoric acid which is undesirable in thecoating solution. A pH in excess of 5.5 often results in thin andpowdery coatings if indeed any coating at all is produced on some steelswhile above a pH of 6.0 no coating at all is produced.

The temperature at which the coating solution may be used rangesanywhere from room temperature to 120 F. Useful coatings have beenproduced at temperatures as low as 40 to 50 F. Higher temperaturesresult in somewhat heavier coatings. The use of temperatures in excessof 120 F. has no deleterious effect upon the quality of the coatingsproduced but higher temperatures cause a degradation of thepyrophosphates to the ortho form which results in a needless and costlywaste of coating chemical as the orthophosphates are inert in thecoating solution of the present invention. 7

Where precleaning has been borderline, it is possible to incorporate inthe coating bath relatively small amounts of surface active agents whichinsure uniformity of coating in such instances.

In general the types of surface active agents which may be used in thisinvention may be of the class of Well known organic detergents of goodwetting and emulsifying power. Examples of wetting agents suitable foruse in this invention are the nonionic detergents such as thederivatives of polyethylene glycols, polyether alcohols and substitutedpolyglycol esters sold under the trade names Igepal (Allied Chemical &Dye Corporation), Lavapon (Rohm & Haas Company) and Nonic 300(Pennsylvania Salt Manufacturing Company);

quaternary ammonium compounds, whichare cationic, sold under the'tradename Hyamine (Rohm & Haas Company; and anionic detergents such assulfonated hydrocarbons and fatty alcohol sulfates sold under theof thepyrophosphating solution 'to be satisfactoryfor Car our purposes. Inaddition there are others which would be useful-the criteria being thatthey should have adequate surface tension depressant values andemulsifying power.

Examples of several pyrophosphate coating solutions which fall under thepurview of this invention are listed below by way of illustrationalthough they are not intended to limit the scope of the inventionexcept as defined in the appended claims.

Example 1 7 Grams Disodium dihydrogen pyrophosphate 20 Sodium chlorate 5Ferric chloride (FeCl .6H O) 0.5

Water, to make 1 liter.

The bath of Example 1 has a pH of 4.2 and produces exceptionally smoothand amorphous pyrophosphate coatings ranging in weight from 40 tomg./sq. ft. when operated at F. and using a S-minutes dip process.

Example 2 V Grams Dipotassium dihydrogen pyrophosphate 20 Sodium nitrite5 Ferric nitrate (Fe(NO .9H O) 1 Lavapon WS-2 1 7 1 Water, to make 1liter.

Reputed to be an alkyl aryl polyether alcohol with 10 ethylene oxidegroups.

The solution of Example 2 had a pH of 4.5 and produced a completelyuniform amorphous pyrophosphate coating on steel surfaces when appliedas a spray process using a 2-minute contact period at 75 F.

Water, to make 1 liter.

The pH of the bath of Example 4 is 4.5. 7 The coating solutions of thisinvention as represented by the foregoing examples may be controlled bypointage titration. Pointage is'commonly defined in the art as thenumber of m1s. N/10 sodium hydroxide which are required to titrate a 10ml. sample of the coatingbath to a permanentphenolphthalein endpoint.Replenishment of a coating bath is readily accomplished on the basis ofthe titration value determined from the control techniques and iseffected by additionsof the same chemicals whichwere used in theoriginal makeeup'of the bath in amounts as dictated by the controlanalyses..

In carrying out the improved process of the present invention, thesurface to be coated should first be cleaned. This step, of itself,forms no part of thepresent invention and may be performed by anyconventional cleaning procedure. For instance, grease and dirt may beremoved by meansof alkali cleaners or an emulsion of a grease solvent.Rust may be removed by. pickling in acid solution. The cleaned work,either wet or dry, may then be treated with a solution of the-presentinvention of which the foregoing are typical examples although manyother formulations may be adopted.

The coating may be formed on the work by immersing the surface to becoated in the solution, or by flowing, spraying or brush painting thesolution on the surface. Also the solution may be wiped on by means of acloth wet with the solution or by any other convenient technique whichwill permit the solution to act upon the work until the desired coatingis produced.

Following the application of the pyrophosphate coating the treated metalsurfaces may be given a final acidulated rinse using a dilute solutionof chromic and/or phosphoric acid. Although this particular step formsno part of the invention and is not necessary in order to secure usefulcorrosion resistant and lasting coatings, it has been observed that afinal passivation of the pyrophosphate coated surfaces tends to increasethe overall corrosion resistance as well as to increase the adherence ofthe final siccative finish.

Inasmuch as the chemical constituents in the pyrophosphate coatingsolutions are depleted during use, it is necessary to replenish thecoating baths at periodic intervals in order to maintain the properbalance of coating materials. We have found that solid admixtures of thecoating chemicals may be used advantageously for both initial solutionmake-up and for replenishing during operation. Admixtures of this typemay contain from 75-85% of a pyrophosphate, from 13 to approximately 25%sodium chlorate or an equivalent amount of an agent capable of oxidizingferrous to ferric iron such as nitrite or per oxygen compound and from0.0008 part to 0.06 part of ferric iron for each part of P 0 Specificexamples of solid admixtures of coating chemical constituents which havebeen found to be useful both for initial solution make-up and forreplenishment are shown below but are presented by way of illustrationonly and are in no way intended to limit the scope of the invention.

In certain industrial operations which employ electrical meteringdevices such as special automated equipment, it is frequently desirableand convenient to employ a concentrated solution of the desiredconstituents in preparing and replenishing the treating baths of ourinvention. This aids in maintaining a homogeneous coating batha matterwhich is of importance where such electrical metering devices forspecial metering equipment are employed. Make-up and replenishingsolutions of this type may be prepared from solid admixtures of thecoating chemicals as represented in Examples 5 and 6 above and a typicalconcentrated solution is prepared by dissolving 1.04 pounds of the solidmixture of Example 5 in 1 gallon of water.

We claim:

1. A composition for use in preparing and replenishing an aqueous metalcoating solution; said composition consisting essentially ofpyrophosphate from the class which consists of sodium, potassium,ammonium and lithium phosphates; accelerating agent from the class whichconsists of chlorates, nitrates and per oxygen compounds; and ferriciron: the quantity of pyrophosphate being from to the quantity ofaccelerating agent being equivalent to from 13 to approximately 25% ofsodium chlorate, and the quantity of ferric iron constituting thebalance on a basis of 0.0008 part to 0.06 part for each part ofpynophosphate present calculated as P 0 2. The method of coating steelwhich comprises treating its surface with an aqueous solution consistingessentially of the following:

Quantity Pyrophosphate from the class consisting of sodium, potassium,ammonium and lithium pyrophosphates from 0.05% to 13% by weight. Ferricion from 0.0008 part for each part of pyrophosphate calculated as P 0 tosaturation of the bath. Accelerating agent from the class which consistsof chlorates, nitrites and peroxygen compounds equivalent in eflfect tofrom 0.2 gram to 10 grams of chlorate per mls. of coating solution.

the solution being maintained at a pH of from 3.2 to 6 and at atemperature not exceeding F.

3. The method of claim 2 wherein the solution is maintained at a pH offrom 3.8 to 5.5.

References Cited in the file of this patent UNITED STATES PATENTS2,067,007 Darsey Jan. 5, 1937 2,514,149 Amundsen July 4, 1950 2,744,555Nicholson et al. May 8, 1956 $2,758,949 Ley et a1. Aug. 14, 19562,856,322 Parson et al. Oct. 14, 1958 2,891,884 Rausch et a1. June 23,1959 FOREIGN PATENTS 517,049 Great Britain Jan. 18, 1940 UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 238L428 June 6 1961Ferdinand P. Heller et ale It is hereby certified that error appears inthe above numbered patent requiring correction and that the said LettersPatent should read as corrected below Column 1, line 58 for solid heavymetal read solid-heavy-metal column 2, line 5 for "power free" readpowder iree line 69 for this read its --=a Signed and sealed this 31stday of October 1961,

(SEAL) Attest:

ERNEST W. SW'IDER DAVID L. LADD Attesting Officer Commissioner ofPatents USCOMM-DC

2. THE METHOD OF COATING STEEL WHICH COMPRISES TREATING ITS SURFACE WITHAN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF THE FOLLOWING: QUANTITYPYROPHOSPHATE FROM THE CLASS CONSISTING OF SODIUM, POTASSIUM, AMMONIUMAND LITHIUM PYROPHOSPHATES FROM 0.05% TO 13% BY WEIGHT. FERRIC ION FROM0.0008 PART FOR EACH PART OF PYROPHOSPHATE CALCULATED AS P2O7 TOSATURATION OF THE BATH. QUANTITY ACCELERATING AGENT FROM THE CLASS WHICHCONSISTS OF CHLORATES, NITRIES AND PEROXYGEN COMPOUNDS EQUIVALENT INEFFECT TO FROM 0.2 GRAM TO 10 GRAMS OF CHLORATE PER 100 MLS. OF COATINGSOLUTION.